I have some of these adaptors, and the 2.5 to 2.1 version..
Can you confirm, to which end, the stamped-in size number refers please? - I would assume it refers to the pin-size in the socket end?
TIA, Phil.

Phil,
This part is described as "DC power plug adaptor that will adapt from a 2.1mm plug to the 2.5mm plug". So you insert a 2.1mm plug into the female socket end of the adaptor, the other end of the adaptor is 2.5mm. 2.1 & 2.5 are the most common sizes used and yes, does refer to the pin diameter, not the barrel diameter.
Hopefully that is clear from my description.